Cathodic protection is a method for controlling corrosion of reinforcing steel in chloride contaminated concrete. Various types of impressed current cathodic protection anodes for reinforced concrete structures have been developed in the past. The anode is one of the most critical components for a cathodic protection system and used to distribute cathodic protection current to the reinforcing steel.
One of the most effective and durable anodes is made of a material which is resistance to corrosion, for example a mixed-metal-oxide (MMO) coated titanium substrate. MMO coated anodes are manufactured by coating a mixture of precious metal oxides on a specially treated precious metal. The coated substrate undergoes multiple thermal treatments at elevated temperatures to gain good bonding properties between the substrate and the coating. Although titanium is widely used as substrate material due to its resistance to corrosion, resistance to chemical attacks and high mechanical strength, other anodes such as tantalum, niobium and zirconium anodes are also used globally for different applications.
Since the first MMO-coated titanium anode was developed in 1984, many concrete structures have been protected using this material. To install the anodes, however, they must be embedded in concrete or cementitious grout. For example, titanium mesh with a concrete overlay, titanium ribbon or ribbon mesh embedded in cemetitious grout in saw-cut slots, or discrete anodes embedded in grout in drilled holes. However, these types of the installation add some burden to the structure and some durability concerns. A useful review of MMO-coated anodes and installation techniques may be found in “Cathodic Protection of Steel in Concrete” By Paul Chess, Taylor & Francis (1998), ISBN 0419230106, the entire content of which is incorporated herein by reference.
The overlay concrete cathodic protection system causes additional dead load to the structure. Frequent disbonding between the overlay and existing concrete is also a serious problem. For the slotted or discrete types of system, the existing concrete must be cut or drilled to install the anodes. However, when the concrete covers over the reinforcing steel are shallow or congested, installation of these types of the systems is not feasible. Even if the anodes are somehow installed in the slots or drilled holes, the vicinity of the reinforcing steel near the anodes may cause an electrical short circuit, resulting in malfunction of the cathodic protection system.